Zirconia-reinforced alumina, also referred to as zirconia-toughened alumina (ZTA), has a matrix composed of alumina (Al2O3) with reinforcements by ZrO2 precipitates for increasing the breaking strength and in particular the self-sharpening capability.
In the alumina (Al2O3) and zirconia (ZrO2) two-material system, in addition to these types of ZTA materials, zirconia materials containing additional alumina, among other materials, are known which thus essentially have a zirconia matrix. Furthermore, materials having amorphous phases or glass phases are known which differ from polycrystalline materials.
Polycrystalline alumina materials may be produced, for example, by pressing a green compact with subsequent sintering to form ceramic. In addition, the formation of polycrystalline materials from a melt is known from DE 26 53 034 A1, for example. The melting of alumina requires high temperatures, the aim in general being to achieve melt temperatures above 2100° C. For this purpose, according to DE 26 53 034 A1 a Higgins furnace is used, in which an electric arc furnace generates the required temperatures. It is proposed to remove the melt not by tipping a crucible, but, rather, to continuously pass cooled, bottle-like bodies through the melt bath, so that material in the melt precipitates due to abrupt cooling on the surface of the bodies, and may subsequently be knocked off by mechanical action. Formation of a eutectic melt composed of Al2O3 and ZrO2 is also described.
However, these types of electric arc furnaces require large amounts of energy. To make effective use of the energy, large charges are generally heated. The production therefore often takes place in areas with inexpensive energy, for example hydroelectric power, or utilizing existing power plant capacities in times of low consumption, for example at night.
DE 699 17 490 T2 describes another method for producing alumina-zirconia abrasive grains, in which a starting material is melted by an electric arc melting process. U.S. Pat. No. 3,891,408 A and US 2008/0028685 A disclose further electric arc melting processes. Charges of several hundred kilograms of the solid starting material are melted and removed in batches. Distances between the zirconia precipitates of approximately 4000 angstrom (400 nm), for example, are achieved in these types of methods.
In electric arc melting processes, the removal is generally carried out by interrupting the electric arc and removing the melt by tipping, for example, when measures as in DE 26 53 034 A1 cited above are not taken. Thus, use of these types of technologies is expensive. In addition, the production of limited charges, for example for producing specific abrasives, is generally ineffective.
WO 2004/071975 A2 describes the production of abrasive grains made of glass-ceramic materials having a triangular shape. For this purpose, solid starting materials containing Al2O3 and additives such as rare earth oxides and zirconia are melted, and the melt is subsequently introduced between two counter-rotating rollers, by means of which the abrasive grains having the desired triangular shape are directly formed from a glass-ceramic material.
The formation of abrasive grains having a suitable shape is also known from EP 6 521 778 B1, for example, which describes a sol-gel method (thickening of a suspension instead of melting the material).
DE 15 19 747 C describes a method for the inductive melting of oxide ceramics, using an inductive melting process. For the initiation, a metallic auxiliary material is initially supplied which ensures the required conductivity, until a portion of the ceramic material is melted and the resulting melt is conductive. The material is melted in a crucible and subsequently solidifies.